1. Field of the Invention
The present invention relates generally to pulse production circuits and, more particularly, to a circuit for producing width-modulated pulses. The invention can be used especially in the field of electrical motor control and, more particularly, for the control of three-phase asynchronous or synchronous controllable motors.
2. Discussion of the Related Art
In industry, there are many uses for three-phase synchronous or asynchronous motors. For example, such motors are commonly used in pump motors. In domestic applications, household appliances are already beginning to be configured to include three-phase motors. Indeed, these motors have the advantage of not requiring switch-over devices and are therefore not subject to the wearing out of brushes experienced in conventional motors.
However, in domestic applications, as well as sometimes in industry, three-phase motors cannot be used because of the unavailability of a mains electrical current in three-phase form. It has therefore been necessary to devise electronic circuits to produce three-phase electrical current from the rectified single-phase AC mains supply. FIG. 1 is a schematic block diagram of an exemplary circuit that performs such a conversion.
A single-phase AC mains supply 1 lets current into a transformer 2 connected to a diode-based full-wave rectifier bridge 3. The rectifier bridge 3 is parallel connected to the terminals of a capacitive circuit 4 comprising, in the illustrated example, two capacitors 5 and 6 and giving a high DC voltage. In one example, this high DC voltage is in the range of 400 volts. A midpoint 7 of the transformer 2 is connected to a midpoint 8 of the capacitors 5 and 6. If it is sought to economize on one of the capacitors 5 or 6, the midpoint 8 no longer exists and, of course, it is no longer connected to the midpoint 7. The midpoint 7 is furthermore connected to an electrical ground 9 of the system.
The capacitors 5 and 6 are series-connected between a high-voltage node 10 and a low-voltage node 11. In the above example, the node 10 is taken to +200 volts with respect to the ground 9 while the node 11 is at -200 volts.
A three-phase motor 12 has three primary windings 13, 14, 15 series-connected with one another by means of connection terminals 16, 17, 18. The terminals 16, 17, 18 are supplied electrically by a switch-over circuit 19. The circuit 19 is itself controlled by a processing circuit 20. The processing circuit 20, in one example, may be supplied from the rectifier circuit 3 in such a way that this set forms a compact device that can easily be positioned against the motor 12. The motor 12 has a rotor 21 with corresponding coils to rotationally drive mechanical loads.
The circuit 19 schematically has three cells that are parallel-connected to one another between the terminals 10 and 11. Each cell has two series-connected transistors that are connected to each other by a midpoint. The midpoints, respectively 22, 23, 24 of each cell, are connected to the terminals 16, 17, 18. The six transistors of the three cells receive, at their gates, control signals produced by the circuit 20. These control signals are typically pulse-width modulated signals.
FIGS. 2a to 2c give a view, in an example, of particular features of production and use of pulse-width modulated control signals. FIG. 2a shows the principle of this production. A first signal generator is used to produce a saw-toothed signal A. Another signal generator is also used to produce a signal B. In the example, the signal B is a sinusoidal signal. It is ensured that the peak-to-peak amplitude of the signals A and B are the same. The frequencies of the signals A and B are substantially different. In one example, the frequency of the signal A will be in the range of 10 KHz while the frequency of the signal B will be in the range of 200 Hz.
In a comparison circuit 27 (FIG. 1), the value of the signal A is permanently compared with the value of the signal B and a pulse C is provided (FIG. 2a) when the signal A is greater than the signal B. It can clearly be seen in FIG. 2a that the pulses C produced in this way are short-duration signals when the signal B is at its maximum and are long-duration signals when signal B is at its minimum. In this way, a modulation of pulses is produced, referred to as a width modulation.
The circuit 20 includes three circuits of this type, only one of which is illustrated to simplify the Figure. Each of these circuits is therefore capable of producing a signal such as the one shown in FIG. 2a. The circuit 20 also has inverter circuits to produce complementary signals of the three signals thus produced. The six resultant signals are applied to the three cells of the circuit 19. Each signal is applied to a transistor of a cell while the complementary signal of that signal is applied to the other transistor of the same cell. It is known that this mode of action makes it possible, by phase-shifting the signals applied to the different cells with respect to one another, to provide a pulsed supply to the terminals 16 to 18 AC voltages of +200 volts to -200 volts. The phase shift leads to the three-phase supply of the motor 12.
The production of the signals A and B in the circuit 20 is generally done by cyclical readings of tables 25, 26 comprising sets of values that are stored in memory. There may be several sets of such tables. At each cycle period of a clock whose pace is set at a frequency f1 or f0 respectively, a new value of the table is read at an address of a word. The address increases gradually from one cycle period to another. When the reading of a last word of the table is arrived at, a loop is set up with the first word and so on and so forth. In one example, the table 25 used to produce the signal B will have 48 values and the table 26 used to produce the saw-toothed signal A will have 256 values.
In practice, rather than returning to an initial reading address when the final reading address of the table has been reached, it is possible to continue the reading by making the address values decrease so that the table is read backwards until the initial value. Then the reading is started again in the original direction and so on and so forth. Consequently, when the signals are symmetrical, the tables may take up less space.
In the example, the frequency f0 of the reading of the table producing the signal A is in the range of 10 MHz. As a result of this, the frequency of the signal A is also in the range of 10 KHz. Indeed, it is necessary to read 256 values upwards and 256 values downwards, giving about 500 values per period of the signal A. In order that the distribution of the pulses of the signal A during a period of the signal B may have sufficient resolution, the number of the signal A pulses is preferably in the range of 50:48 (which is a multiple of two). This naturally leads to an upper limit frequency for the signal B equal to 200 Hz (10,000/50=200). In view of the fact that it is sought to create signals which are phase shifted by 120.degree., the table 25 is read at each time with two shifts for the reading, each time, of three values to be compared with the value read in the table 26.
FIG. 2b shows the same elements as FIG. 2a, the only difference being that in FIG. 2b the frequency of the signal B is greater than that of FIG. 2a. The mode of comparison of the signals A and B in the comparator 27 is shown on the bottom of FIG. 2b.
FIG. 2c shows what could happen if the instant of the change of state of the signal B is not synchronized with the instants of the changes of state of the signal A. FIG. 2c gives an enlarged view of the part of FIG. 2a surrounded by dashes. In FIG. 2c, the growth of the signal A is faster than the growth of the signal B owing to the difference in frequency referred to here above. Rather than having only one instant when the signal A becomes greater than the signal B, in certain cases, signal B may, for a very short duration in the range of one or two cycle periods at the frequency f0, be smaller than the signal A. After this period, signal B may then be greater during one more cycle period before again becoming smaller than signal A for a longer duration. The presence of stray pulses such as a pulse 28 shown in FIG. 2c has led those skilled in the art to synchronize the signals with the frequencies f0 and f1 for controlling the reading of the tables 25 and 26.
For three-phase synchronous motors, the only way to change the speed of rotation of the motor is to modify the frequency of the signal B. However, to meet the conditions of synchronization dictated by the disorderly conditions described above and illustrated in FIG. 2c, it has been accepted as a necessity that the signals must be synchronized with the frequency f0 and f1 that enable the production of the signals A and B. In practice, this leads to a situation where f1 is a sub-multiple of f0. This leads to a situation where the modification of the frequency f1 can progress only by operations of division by integers. In the illustrated example, it is thus possible to make f1 vary by 200 Hz to 100 Hz and then 66 Hz and then 50 Hz, etc. However, these frequency jumps are far too great if it is sought to regulate the speed of the motors more precisely. Indeed, if it is arranged that the signal at 200 Hz leads to a motor speed of the order of 1,000 rpm, then the passage to 100 Hz will lead to a modification of this speed into a speed of 500 rpm, without any possibility of obtaining intermediate speeds at 825 rpm, 790 rpm, etc.
One conventional approach has been to duplicate the reading of certain words in the memory table that enables the production of the signal B. By reading the same word twice, for example, the period of the signal B lasts one more cycle period, namely 49 cycle periods instead of 48 cycle periods. This period is modified by about 2%, which is an acceptable adjustment. The problem, however, is to choose those memory words of the memory that must be read several times. Indeed, if the operation is limited to the reading of the first words of the tables several times (namely those words whose address is close to the beginning of the table), a distortion of the equivalent alternating signal produced for the signal B is prompted. This distortion leads to advances and delays of phases of the supply of the windings 13, 14, 15 which are ultimately very poorly supported by the motor 12. This in turn leads to an irregular wearing-out of its rotation bearings. Furthermore, these distortions reverberate as electrical parasites are reinjected into the network 1.
There are known ways, however, of distributing the addresses of the words to be read several times among the possible addresses of the words of the memory 25. However, this distribution implies the use of a microprocessor and of a recorded microprogram whose power, and hence cost, have no relation to the expected cost of the motor 12 and its electronic control circuit. Indeed, for motors for domestic use, the cost of the motor coil and control circuit should be minimal. It is therefore impossible to control a motor of this kind with a microprocessor-based circuit whose cost is significant.